As a technique for recording and reproducing digital data, a data recording technique is available which uses an optical disk (including a magneto-optical disk) such as, for example, a CD (Compact Disc), an MD (Mini-Disc) and a DVD (Digital Versatile Disc) as a recording medium. The optical disk is a general term for recording media of the type wherein laser light is illuminated upon a disk formed from a thin metal plate protected with plastic and a signal is read from a change in reflected light from the disk.
Optical disks are divided into those of the type which can be used only for reproduction as known as, for example, a CD, a CD-ROM and a DVD-ROM and those of the type on which user data is recordable as known as an MD, a CD-R, a CD-RW, a DVD-R, a DVD-RW, a DVD+RW and a DVD-RAM. For optical disk of the recordable type, a magneto-optical recording method, a phase change recording method, a pigment film change recording method or the like is used to allow recording of data. The pigment film change recording method is also called write-once recording method and allows data recording only once but does not allow re-writing, and therefore, it is suitably used for data storage applications and so forth. Meanwhile, the magneto-optical recording method and the phase change recording method allow rewriting of data and are utilized for various applications beginning with recording of various kinds of contents data such as music, videos, games, application programs and so forth.
Further, a high density optical disk called DVR (Data & Video Recording) disk has been developed in recent years and exhibits a significant increase in capacity.
In order to record data onto a disk which allows recording by the magneto-optical recording method, pigment film change recording method or phase change recording method, guide means for performing tracking of a data track is required. To this end, a groove is formed in advance as a pregroove, and the groove or a land (a location of a trapezoidal cross section positioned between grooves) is used as a data track.
Also it is necessary to record address information in order to allow data to be recorded at a predetermined position on a data track. Such address information is sometimes recorded by wobbling a groove.
In particular, while a track onto which data is to be recorded is formed in advance, for example, as a pregroove, side walls of the pregroove are wobbled in accordance with address information.
Where side walls of a pregroove are wobbled in this manner, upon recording or upon reproduction, an address can be read from the wobbling information obtained as reflected light information. Thus, even if, for example, pit data or the like representative of addresses is not formed on the track in advance, data can be recorded or reproduced at a desired position.
Where address information is added as a wobbling groove in this manner, for example, if address areas are provided discretely on tracks, then it is unnecessary to record addresses, for example, as pit data, and the recording capacity for actual data can be increased by an amount by which the address areas can be eliminated.
It is to be noted that absolute time (address) information represented by such a wobbling groove as described above is called ATIP (Absolute Time In Pregroove) or ADIP (Address In Pregroove).
Incidentally, particularly with a rewritable disk, it is necessary to record, in addition to address information and information (user data) which is recorded and reproduced by a user, attributes of the disk and recording and reproduction powers, pulse information and so forth as numerical values for use for control as additional information in advance on the disk similarly to the address information. For such additional information, a high degree of reliability is required.
The reason why a high degree of reliability is required is that, for example, if attributes or additional information for control is not obtained accurately, then an apparatus on the user side cannot execute such a control operation for establishing optimum recording conditions correctly.
As a method of recording such information on a disk in advance, it is known to form emboss pits on a disk.
However, if it is intended to achieve high density recording and reproduction onto and from an optical disk, the prerecording method by emboss pits is disadvantageous.
In order to achieve high density recording and reproduction onto and from an optical disk, it is necessary to form the groove with a reduced depth. With a disk produced with a stamper such that a groove and emboss pits are formed at a time, it is very difficult to form the groove and the emboss pits with different depths from each other. Therefore, it cannot be avoided to make the depth of the emboss pits equal to the depth of the groove.
However, where the depth of the emboss pits is small, there is a problem that a signal of a good quality cannot be obtained from the emboss pits.
For example, where an optical system including a laser diode of a wavelength of 405 nm and an objective lens of NA=0.85 is used and phase change marks are recorded and reproduced with a track pitch of 0.32 μm and a linear density of 0.12 μm/bit onto and from a disk having a cover (substrate) thickness of 0.1 mm, a capacity of 23 GB (GigaBytes) can be recorded onto and reproduced from an optical disk having a diameter of 12 cm.
In this instance, the phase change marks are recorded onto and reproduced from the groove formed spirally on the disk, and in order to suppress the medium noise to achieve high density, preferably the depth of the groove is set to approximately 20 nm, that is, λ/13 to λ/12 with respect to the wavelength λ.
On the other hand, in order to obtain a signal of a good quality from the emboss pits, preferably the depth of the emboss pits is λ/8 to λ/4. After all, a good solution to a common depth to the groove and the emboss pits cannot be obtained.
From such a situation as just described, a method of recording necessary additional information in advance in place of emboss pits is demanded. Besides, it is demanded to record the additional information with a high degree of reliability.